1. Field of the Invention
This invention relates to press-to-actuate type switches. More particularly, it relates to electromechanical keyboards of the type commonly used on office equipment such as personal computers.
2. Description Of the Related Art
Alpha-numeric keyboards of the type used for personal computers and the like typically comprise an electrical switch, a resilient member such as a spring or a rubber dome, a plunger, a keycap, and a switch housing which provides a guide for the action of the plunger. Most commonly, the keycap is attached to the upper end of the plunger (or the keycap and plunger are fabricated as a single unit) and the lower end of the plunger bears against the resilient member which provides a resistance to the downward action of the plunger and returns the keycap to its rest position (up position) when finger pressure on the keycap is released. In many keyboards, the under or lower surface of the resilient member contacts a membrane-type electrical switch when the resilient member is compressed by depressing the keycap. This switch action is electrically communicated to the host device (e.g., a computer) and is interpreted as a key actuation.
The "feel" of a keyboard is determined by a number of factors. A principal factor is the fit of the plunger within the switch housing (i.e., the dimensions and tolerances thereof). A common plunger and switch housing design of the prior art is illustrated in cross section in FIG. 1. Keycap 10 is attached to plunger 30 which is inserted into opening 60 of switch housing 20. Most commonly, plunger 30 and opening 60 will be generally circular in a cross section perpendicular to that shown in FIG. 1. The opening 60 in switch housing 20 which receives plunger 30 is often referred to in the art as a "chimney". For purposes of illustration, the taper of chimney 60 and plunger 30 are exaggerated in FIG. 1. In a design of the type shown, the diameter of plunger 30 at line 40 is greater than that at line 50. Conversely, the diameter of chimney 60 is less at line 40 than at line 50.
The use of tapers (or "drafts" or "draft angles") on keyboard members such as those illustrated in FIG. 1 have a number of advantages. Since these parts are most commonly fabricated of plastics by injection molding techniques, a draft angle facilitates the release of the part from the mold at the end of the molding cycle. Moreover, the taper of the plunger enables one to easily insert the plunger into the chimney of the switch housing from the upper surface of the switch housing thereby facilitating the assembly of the keyboard.
A particular disadvantage of this sort of plunger/chimney design is that the "fit" of the plunger is loosest (i.e., the clearance between the plunger and chimney walls is greatest) when the key is the rest (i.e., nondepressed) position. This contributes to a "loose feel" or "wobble" of the keycap when a typist's fingers are resting on the "home keys" of the keyboard. Users typically associate this "loose feel" with low keyboard quality; hence, it is undesirable. The present invention is directed, in part, to solving this problem.
Keycap/plunger combinations of the type illustrated in FIG. 1 are commonly secured in chimney 60 by means of tabs or barbs (not shown) which are molded into the plunger with corresponding tab-engaging surfaces molded into the switch housing. Such devices provide a positive "up stop" for the key and prevent the plunger from being pulled out of the chimney. However, when an up stop engages, it usually produces a sound which is audible to users. It has been found that users of computer keyboards (particularly touch typists) find this sound objectionable. While users express a preference for audible feedback upon key switch activation, a sound produced by a keyboard upon key release is undesirable. It is contemplated that this sound is undesirable because it occurs at the "wrong" time in the keystroke cycle. The present invention solves these problems.